Effect of wavelength and liquid on formation of Ag, Au, Ag/Au nanoparticles via picosecond laser ablation and SERS-based detection of DMMP

• Sree Satya Bharati Moram,
• Chandu Byram and
• Venugopal Rao Soma

Beilstein J. Nanotechnol. 2024, 15, 1054–1069, doi:10.3762/bjnano.15.86

• efficiency and yield of the LASiS process [4][50]. The study reported by Shukri et al. [51] pointed out a size reduction in Au NPs from 19 to 12 nm by decreasing the input laser wavelengths, changing from 1064 to 532 nm in DW. Solati et al. [52] observed a significant increase in the mean size of Ag NPs from

• the variations of NP particle size or size dispersion. The peak shift is mainly correlated with the size and shape of NPs and their surrounding medium. Naderi-Samani et al. [56] reported the synthesis of Ag NPs by nanosecond laser ablation in different aqueous solutions: water, acetone

• , cetyltrimethylammonium chloride (CTAC), polyvinylpyrrolidone (PVP), and sodium dodecyl sulfate (SDS). Their outcomes revealed that the productivity of Ag NPs was higher in acetone, followed by CTAC, water, PVP, and SDS. The order of NP formation efficiency was reported to be acetone >CTAC >water >PVP >SDS. Morphological

Atomistic insights into the morphological dynamics of gold and platinum nanoparticles: MD simulations in vacuum and aqueous media

• Evangelos Voyiatzis,
• Eugenia Valsami-Jones and
• Antreas Afantitis

Beilstein J. Nanotechnol. 2024, 15, 995–1009, doi:10.3762/bjnano.15.81

• shape was looked into by Holec et al. [48], while Martin et al. considered silver NPs [49]. A comparative study of surface disorder in Au and Ag NPs upon cooling was carried out by Agudelo-Giraldo et al. [50]. Chushak and Bartell considered the structural modifications upon freezing of several molten Au

• surfaces of the large NPs. This is in agreement with previous findings for Ag NPs with a similar diameter range [49]. It should be noted that the considered variations in the NP size are rather subtle and below detection for current analytical capabilities [85]. There are slight disparities between Au and

Potential of a deep eutectic solvent in silver nanoparticle fabrication for antibiotic residue detection

• Le Hong Tho,
• Bui Xuan Khuyen,
• Ngoc Xuan Dat Mai and
• Nhu Hoa Thi Tran

Beilstein J. Nanotechnol. 2024, 15, 426–434, doi:10.3762/bjnano.15.38

• (Ag NPs), but the potential of DESs in Ag NP synthesis was neither considered nor studied carefully. In this study, we present a novel strategy to fabricate Ag NPs in a DES (Ag NPs-DES). The DES composed of ᴅ-glucose, urea, and glycerol does not contain any anions to precipitate with Ag+ cations. Our

• Ag NPs-DES sample is used in a surface-enhanced Raman scattering (SERS) sensor. The two analytes for SERS quantitation are nitrofurantoin (NFT) and sulfadiazine (SDZ) whose residues can be traced down to 10−8 M. The highest enhancement factors (EFs) are competitive at 6.29 × 107 and 1.69 × 107 for

• synthesis of nanomaterials for biosensor substrate construction. Keywords: Ag NPs; antibiotic residue; deep eutectic solvents; potential; SERS; Introduction Surface-enhanced Raman scattering (SERS) is a ubiquitous technology for detecting and tracing substances, applied in various kinds of sensors. The

Silver-based SERS substrates fabricated using a 3D printed microfluidic device

• Phommachith Sonexai,
• Minh Van Nguyen,
• Bui The Huy and
• Yong-Ill Lee

Beilstein J. Nanotechnol. 2023, 14, 793–803, doi:10.3762/bjnano.14.65

• catalytic chemistry, where they have been used to develop localized surface plasmon resonance (LSPR) and SERS substrates [8]. For example, Ag NPs yield a strong SERS effect at relatively low cost. However, an issue often encountered in synthetic approaches is the non-uniformity of the Ag NPs. Homogeneous Ag

• developed system. The substrate was prepared from Ag NPs by decorating the surface of porous silicon with Ag NPs using self-assembled monolayers (SAMs). The SAM method offers precise control, versatility, simplicity, stability, and compatibility, making it a valuable technique for surface modification in

• various scientific and technological applications. The Ag NPs were synthesized using a droplet-based microfluidic device and a stereolithographic 3D printing method. The microfluidic device was optimized to produce uniform droplets, within which silver nitrate was reduced by sodium borohydride. This

Quercetin- and caffeic acid-functionalized chitosan-capped colloidal silver nanoparticles: one-pot synthesis, characterization, and anticancer and antibacterial activities

• Akif Hakan Kurt,
• Elif Berna Olutas,
• Fatma Avcioglu,
• Hamza Karakuş,
• Mehmet Ali Sungur,
• Cansu Kara Oztabag and
• Muhammet Yıldırım

Beilstein J. Nanotechnol. 2023, 14, 362–376, doi:10.3762/bjnano.14.31

• characterization of quercetin- and caffeic acid-functionalized chitosan-capped colloidal silver nanoparticles (Ch/Q- and Ch/CA-Ag NPs), and their antibacterial and anticancer activities. The formation of Ch/Q- and Ch/CA-Ag NPs has been confirmed by ultraviolet–visible (UV–vis) spectroscopy, Fourier-transform

• infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The characteristic surface plasmon resonance (SPR) absorption band has been found at 417 and 424 nm for Ch/Q- and Ch/CA-Ag NPs, respectively. The formation of a chitosan shell comprising quercetin and caffeic acid, which surround the

• colloidal core Ag NPs, was confirmed by UV–vis, and FTIR analyses, and monitored by TEM microscopy. The size of nanoparticles has been determined as 11.2 and 10.3 nm for Ch/Q- and Ch/CA-Ag, respectively. The anticancer activity of Ch/Q- and Ch/CA-Ag NPs has been evaluated against U-118 MG (human

Supramolecular assembly of pentamidine and polymeric cyclodextrin bimetallic core–shell nanoarchitectures

• Alexandru-Milentie Hada,
• Nina Burduja,
• Marco Abbate,
• Claudio Stagno,
• Guy Caljon,
• Louis Maes,
• Nicola Micale,
• Massimiliano Cordaro,
• Angela Scala,
• Antonino Mazzaglia and
• Anna Piperno

Beilstein J. Nanotechnol. 2022, 13, 1361–1369, doi:10.3762/bjnano.13.112

• impressive stability over time and over sterilization procedures, probably due to the colocalization of reduction and nucleation/growing sites during their formation. The stability was maintained or was found to be even higher in PolyCD-capped Au@Ag BMNPs, whereas monometallic Ag NPs produced from PolyCD in

• of NPs and that this step is preparatory for the subsequent deposition of the silver shell. Conversely, in the case of monometallic Ag NPs, the reduction of silver ions (usually mediated by ascorbic acid) takes place at sites which are not involved in the nucleation/growing process leading to poorly

• stable Ag NPs. In the framework of our research on BMNPs, we collected several experimental data, including FTIR spectra, which suggested strong interactions between PolyCD and Au@Ag BMNPs [14]. In this paper we partially clarify the role of PolyCD in the formation/stabilization of Au@Ag BMNPs by

Zinc oxide nanostructures for fluorescence and Raman signal enhancement: a review

• Ioana Marica,
• Fran Nekvapil,
• Maria Ștefan,
• Cosmin Farcău and
• Alexandra Falamaș

Beilstein J. Nanotechnol. 2022, 13, 472–490, doi:10.3762/bjnano.13.40

• allowed for the decoration of vertically aligned cone-shaped ZnO nanorods with Ag NPs on their sides and their top ends [35], and magnetron sputtering [7][36]. Electron beam evaporation of, for example, a 30 nm Au layer on ZnO nanopillars arrays [37] or of ZnO-elevated Au dimer nanostructures (Figure 2a,b

• include hydrothermal method [42], controlled wet chemistry using spin coating, which can deposit, for example, uniform Ag NPs onto ZnO nanowhiskers and nanocolumns [43], photochemical deposition, which allows the formation of different Au shapes depending on the type of additive added into the

• photochemical reduction [32], pulsed laser-induced photolysis [38], or controlled decoration with Ag NPs using an electroless plating technique [44]. Photochemical synthesis permits the control of nucleation and growth rate without using organic additives. Xu et al. employed laser irradiation of ZnO nanorods in

Engineered titania nanomaterials in advanced clinical applications

• Padmavati Sahare,
• Paulina Govea Alvarez,
• Juan Manual Sanchez Yanez,
• Gabriel Luna-Bárcenas,
• Samik Chakraborty,
• Sujay Paul and
• Miriam Estevez

Beilstein J. Nanotechnol. 2022, 13, 201–218, doi:10.3762/bjnano.13.15

• (GNP2) presented good osseointegration [61]. In another recent study, TiO2 nanotubes (TNT) were grown on the surface of medical-grade titanium alloy and then coated with silver nps (Ag nps) to improve the antimicrobial properties of the implants. Moreover, to avoid direct contact of Ag nps with human

A comprehensive review on electrospun nanohybrid membranes for wastewater treatment

• Senuri Kumarage,
• Imalka Munaweera and
• Nilwala Kottegoda

Beilstein J. Nanotechnol. 2022, 13, 137–159, doi:10.3762/bjnano.13.10

• incorporated Ag NPs into the same composite of PVA, PAA, and carboxyl-functionalized GO. The resulting membrane showed a remarkable photocatalytic decomposition efficiency for MB even after eight catalytic cycles at room temperature [81]. A novel functionalized HNT-incorporated PVDF nanofiltration membrane has

Sputtering onto liquids: a critical review

• Anastasiya Sergievskaya,
• Adrien Chauvin and
• Stephanos Konstantinidis

Beilstein J. Nanotechnol. 2022, 13, 10–53, doi:10.3762/bjnano.13.2

• nuclei of the final NPs [94][95]. Because silver clusters absorb light at different wavelengths than the surface plasmon resonance (SPR) band of Ag NPs (which is used for monitoring of NP growth kinetics by UV–vis spectroscopy) one can spot the induction period on the sigmoidal kinetic curves. Noteworthy

• is the important contribution by Huang et al. [96], who proved the autocatalytic nature of NP growth by an easy experimental test. They introduced Ag NPs to a solution containing Ag ions and a reducer. This led to a significant increase in the process rate due to the reduction of Ag ions on the NP

• of Ag NPs from 5.7 to 11 nm. In the case of Au sputtering onto 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HyEMI-BF4), changing the current from 10 to 40 mA resulted in changing the size of Au NPs from 2.7 to 4.7 nm [126]. Studying MS of Cu onto pentaerythritol ethoxylate (PEEL) showed

Self-assembly of amino acids toward functional biomaterials

• Huan Ren,
• Lifang Wu,
• Lina Tan,
• Yanni Bao,
• Yuchen Ma,
• Yong Jin and
• Qianli Zou

Beilstein J. Nanotechnol. 2021, 12, 1140–1150, doi:10.3762/bjnano.12.85

• nanoparticles (Ag NPs) hold great promise due to their broad-spectrum and robust antimicrobial properties [65]. The main mechanism is that Ag nanoparticles diffuse into cells and destroy cell walls [66]. However, Ag NPs are cytotoxic, which limits their application [67]. Song et al. [68] developed a broad

Modification of a SERS-active Ag surface to promote adsorption of charged analytes: effect of Cu²⁺ ions

• Bahdan V. Ranishenka,
• Andrei Yu. Panarin,
• Irina A. Chelnokova,
• Sergei N. Terekhov,
• Peter Mojzes and
• Vadim V. Shmanai

Beilstein J. Nanotechnol. 2021, 12, 902–912, doi:10.3762/bjnano.12.67

• the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the intensity of surface-enhanced Raman scattering (SERS). For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently

• the Ag NPs. Literature analysis reveals that many authors use cationic organic dyes as SERS probing analytes [18][25]. In contrast, practically interesting biological molecules are mostly negatively charged. In 2015, the authors of [18] pointed out a possibility to prepare positively charged Ag NPs to

• analyze anionic analytes, which are hardly detectable by the Ag NPs prepared by common protocols. They used thiocholine to create a strong positive charge on the Ag NP surface. However, the solution of the charge problem was not so simple because citrate ions, which are used in many common protocols

Silver nanoparticles nucleated in NaOH-treated halloysite: a potential antimicrobial material

• Yuri B. Matos,
• Rodrigo S. Romanus,
• Mattheus Torquato,
• Edgar H. de Souza,
• Rodrigo L. Villanova,
• Marlene Soares and
• Emilson R. Viana

Beilstein J. Nanotechnol. 2021, 12, 798–807, doi:10.3762/bjnano.12.63

• 10.3762/bjnano.12.63 Abstract Despite all recent advances in medical treatments, infectious diseases remain dangerous. This has led to intensive scientific research on materials with antimicrobial properties. Silver nanoparticles (Ag-NPs) are a well-established solution in this area. The present work

• studied the nucleation of silver on halloysite substrates modified by chemical treatment with NaOH. The resulting stabilized Ag-NPs were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The nucleation was characterized by thermogravimetric

• analysis and differential scanning calorimetry. The antimicrobial properties of the Ag-NPs were investigated against E. coli and S. aureus. The potential of the Ag-NPs for industrial application was tested by dispersing them into low-density polyethylene. The importance of the chemical affinity between

Silver nanoparticles induce the cardiomyogenic differentiation of bone marrow derived mesenchymal stem cells via telomere length extension

• Khosro Adibkia,
• Ali Ehsani,
• Asma Jodaei,
• Ezzatollah Fathi,
• Raheleh Farahzadi and
• Mohammad Barzegar-Jalali

Beilstein J. Nanotechnol. 2021, 12, 786–797, doi:10.3762/bjnano.12.62

• possible combination of stem cells and nanotechnology in the treatment of diseases. This study aims to investigate the in vitro effect of silver nanoparticles (Ag-NPs) on the cardiomyogenic differentiation of bone marrow-derived mesenchymal stem cells (BM-MSCs) through detection of cardiac markers. For

• this purpose, MSCs were isolated from bone marrow resident and differentiated to the cardiac cells using a dedicated medium with Ag-NPs. Also, the cardiomyogenic differentiation of BM-MSCs was confirmed using immunocytochemistry. Then, real-time PCR and western blotting assay were used for measuring

• absolute telomere length (TL) measurement, and gene and protein assessment of the cells, respectively. It was found that 2.5 µg/mL Ag-NPs caused elongation of the telomeres and altered VEGF, C-TnI, VWF, SMA, GATA-4, TERT, and cyclin D protein and gene expression in the cardiomyogenically differentiated BM

A review on the biological effects of nanomaterials on silkworm (Bombyx mori)

• Sandra Senyo Fometu,
• Guohua Wu,
• Lin Ma and
• Joan Shine Davids

Beilstein J. Nanotechnol. 2021, 12, 190–202, doi:10.3762/bjnano.12.15

• ), graphene oxide, silver nanoparticles (Ag NPs) [24][25][26], quantum dots, and superparamagnetic particles [27] have been reported to have antibacterial properties against Streptococcus mutans [28] and Xanthomonas perforans, antifungal properties against Fusarium oxysporum [27] and Fusarium graminearum [29

• uptake of carbon black and single-walled nanotubes caused impairment in the locomotor function of these flies. Demir et al. [42] reported that Ag NPs (0.1, 1, 5, and 10 mM) induced genotoxicity in the wing spot assay of fruit flies via somatic recombination. Carmona et al. [43] reported cytotoxic effects

• ] reported that fruit flies exposed to 20 mg/L of Ag NPs were unable to complete their lifecycle. However, after a long-term exposure to Ag NPs, later generations of flies had fecundity levels similar to those of the fruit flies from the control group. Its natural habitat, rapid life cycle (≈4 days

Antimicrobial metal-based nanoparticles: a review on their synthesis, types and antimicrobial action

• Matías Guerrero Correa,
• Fernanda B. Martínez,
• Cristian Patiño Vidal,
• Camilo Streitt,
• Juan Escrig and
• Carol Lopez de Dicastillo

Beilstein J. Nanotechnol. 2020, 11, 1450–1469, doi:10.3762/bjnano.11.129

• allows for nanoparticles to be synthesized directly from a metal source. Jung et al., for example, obtained Ag NPs in various sizes, ranging from 6 to 21 nm. However, this technique has the disadvantage of consuming a significant amount of energy and it requires a large lab space for the experimental

• intracellularly or in the extracellular milieu, where the biomolecules released from the cells are located. As an example of the latter, Pseudomonas strutzeri bacteria can successfully generate Ag NPs extracellularly [84]. Conversely, the bioreduction of iron followed by the precipitation of an oxide, which is

• bacteria (inhibition zone diameter of E. coli: 22 ± 0.86 mm) and Gram-positive bacteria (inhibition zone diameter of B. subtilis: 23 ± 0.9 mm) [88]. Bio-reduction of silver nitrate with Parkia speciosa leaf extract generated spherical Ag NPs with an average particle size of 31 nm [89]. A major

Photothermally active nanoparticles as a promising tool for eliminating bacteria and biofilms

• Mykola Borzenkov,
• Piersandro Pallavicini,
• Angelo Taglietti,
• Laura D’Alfonso,
• Maddalena Collini and
• Giuseppe Chirico

Beilstein J. Nanotechnol. 2020, 11, 1134–1146, doi:10.3762/bjnano.11.98

• results regarding antibacterial activity. The advances in the field of nanomaterials exhibiting antibacterial activity are well summarized in recent reviews [1][7][8][9]. In particular, the antibacterial properties of silver nanoparticles (Ag NPs) and Ag-NP-based polymeric materials are the most

• intensively studied within the broad spectrum of existing nanomaterials [10][11][12][13][14][15]. However, the low stability (even at physiological pH levels) and the potential toxicity of Ag NPs limit their application [7][16]. Moreover, the results regarding the efficiency of Ag NPs in preventing biofilm

• excitation [6]. Later, these authors introduced a new photo-responsive antibacterial surface preparation approach by combining the on-demand photo-switchable hyperthermal action and the sustained biocidal Ag+ release from an upper layer of Ag NPs by sequentially grafting and segregating gold nanostars and Ag

Gram-scale synthesis of splat-shaped Ag–TiO₂ nanocomposites for enhanced antimicrobial properties

• Mohammad Jaber,
• Asim Mushtaq,
• Kebiao Zhang,
• Jindan Wu,
• Dandan Luo,
• Zihan Yi,
• M. Zubair Iqbal and
• Xiangdong Kong

Beilstein J. Nanotechnol. 2020, 11, 1119–1125, doi:10.3762/bjnano.11.96

• specifically, Ag NPs have been widely used in many fields, such as dental filling, wound dressing, water treatment and textile fabrics [12][13]. However, issues have been raised concerning Ag-associated genotoxicity and cytotoxicity in human cells [14]. To solve these toxicity problems, nanocomposite materials

• nanoparticles (50 nm) appeared at the surface of theTiO2 nanospheres (Figure 2b,c). It was observed that the amount of the Ag NPs increased when the AgNO3 concentration increased. Figure 2d,f shows the EDS spectra. As expected, the EDS results identified Ag, Ti and O as the components present in the prepared

• different concentrations of Ag. Characterization techniques such as XRD, SEM, EDS and FTIR corroborated the expected results and showed that the Ag–TiO2 NCs exhibit good biocompatibility. The amount of small Ag metal particles at the surface of the TiO2 nanospheres and Ag NPs increased when the Ag

Pulsed laser synthesis of highly active Ag–Rh and Ag–Pt antenna–reactor-type plasmonic catalysts

• Kenneth A. Kane and
• Massimo F. Bertino

Beilstein J. Nanotechnol. 2019, 10, 1958–1963, doi:10.3762/bjnano.10.192

• ]. Nonradiative absorption results in the generation of energetic charge carriers [11][12] and chemical bonds can be activated with the energy created by the charge carriers, documented by Christopher et al. who reported partial oxidation reactions on plasmonic Ag NPs induced by resonant light of relatively low

A silver-nanoparticle/cellulose-nanofiber composite as a highly effective substrate for surface-enhanced Raman spectroscopy

• Yongxin Lu,
• Yan Luo,
• Zehao Lin and
• Jianguo Huang

Beilstein J. Nanotechnol. 2019, 10, 1270–1279, doi:10.3762/bjnano.10.126

• nanoparticles (Ag-NPs) onto the surfaces of the cellulose nanofibers (NFs) in ordinary laboratory filter paper by means of the one-step silver mirror reaction. Both size and density of the of the silver nanoparticles on the substrates could be controlled. This paper-based silver-nanoparticle/cellulose-nanofiber

• in all the experiments was purified by using a Milli-Q Advantage A10 system (Millipore, Bedford, MA, USA) with a resistivity higher than 18.2 MΩ·cm. Fabrication of the paper-based Ag-NP/cellulose-NF composites The silver nanoparticles (Ag-NPs) were deposited onto the surface of the cellulose

Effects of gold and PCL- or PLLA-coated silica nanoparticles on brain endothelial cells and the blood–brain barrier

• Aniela Bittner,
• Angélique D. Ducray,
• Hans Rudolf Widmer,
• Michael H. Stoffel and
• Meike Mevissen

Beilstein J. Nanotechnol. 2019, 10, 941–954, doi:10.3762/bjnano.10.95

• . None of the NPs investigated impaired expression, integrity or functionality of TJs. In accordance with our findings, PEGylated Au-NPs did not alter TJ expression, TEER or Papp of a co-culture model consisting of PBECs and SH-SY5Y [24]. Copper and Ag-NPs on the other hand led to an increase in

Fabrication of silver nanoisland films by pulsed laser deposition for surface-enhanced Raman spectroscopy

• Bogusław Budner,
• Mariusz Kuźma,
• Barbara Nasiłowska,
• Bartosz Bartosewicz,
• Malwina Liszewska and
• Bartłomiej J. Jankiewicz

Beilstein J. Nanotechnol. 2019, 10, 882–893, doi:10.3762/bjnano.10.89

• resonance of Ag NPs. The small shift of the plasmon resonance peak may be related to rather small variations of the size of particles between samples obtained using various deposition process parameters. Then reflectance increases and reaches a maximum in the range of 450 to 520 nm and decreases again

Electrospun one-dimensional nanostructures: a new horizon for gas sensing materials

• Muhammad Imran,
• Nunzio Motta and
• Mahnaz Shafiei

Beilstein J. Nanotechnol. 2018, 9, 2128–2170, doi:10.3762/bjnano.9.202

• °C with response/recovery times of 75 s/12 s [223]. Importantly, the operating temperature of ZnO–SnO2 composite NTs decreases from 215 °C to 140 °C by decorating with Ag NPs using a seed-mediated growth method [158]. CuO/SnO2 mixed NFs synthesized by a double needle electrospinning technique have

A visible-light-controlled platform for prolonged drug release based on Ag-doped TiO₂ nanotubes with a hydrophobic layer

• Caihong Liang,
• Jiang Wen and
• Xiaoming Liao

Beilstein J. Nanotechnol. 2018, 9, 1793–1801, doi:10.3762/bjnano.9.170

• length of the first layer was 400 nm (which is shorter than the length of the second layer at 2.06 μm) [15]. Elemental Ag was detected in the EDS spectrum of Ag-TNTs (Figure 1e) on the surface of the TNTs, suggesting that the Ag NPs were successful doped into the TiO2 nanotubes. From Figure 1d it can be

Review on nanoparticles and nanostructured materials: history, sources, toxicity and regulations

• Jaison Jeevanandam,
• Ahmed Barhoum,
• Yen S. Chan,
• Alain Dufresne and
• Michael K. Danquah

Beilstein J. Nanotechnol. 2018, 9, 1050–1074, doi:10.3762/bjnano.9.98

• production methods for these carbon-based materials fabrication (except carbon black) [8]. (ii) Inorganic-based nanomaterials: These NMs include metal and metal oxide NPs and NSMs. These NMs can be synthesized into metals such as Au or Ag NPs, metal oxides such as TiO2 and ZnO NPs, and semiconductors such as

• found that the Lycurgus Cups contain Ag–Au alloy NPs, with a ratio of 7:3 in addition to about 10% Cu [29]. Later, red and yellow colored stained glass found in medieval period churches was produced by incorporating colloidal Au and Ag NPs, respectively [25]. During the 9th century, Mesopotamians

• and blue colors under particular reflection conditions. TEM analysis of these ceramics revealed a double layer of Ag NPs (5–10 nm) in the outer layer and larger ones (5–20 nm) in the inner layer. The distance was observed to be constant at about 430 nm in between two layers, giving rise to